The present invention relates to air plasma spray (APS) thermal barrier coatings (TBCs) such as are commonly applied to articles for use in high temperature environments. More specifically, the present invention comprises APS TBCs having a coherent, continuous columnar grain microstructure and a preferred vertical crack pattern which enhance the physical and mechanical properties of these coatings in ways which are intended to improve their resistance to spalling in cyclic high temperature environments.
APS TBCs are well known, having been used for several decades. They are typically formed from ceramic materials capable of withstanding high temperatures and are applied to metal articles to inhibit the flow of heat into these articles. It has long been recognized that if the surface of a metal article which is exposed to a high temperature environment is coated with an appropriate refractory ceramic material, then the rate at which heat passes into and through the metal article is reduced, thereby extending its applicable service temperature range, service longevity, or both.
Prior art APS TBCs are typically formed from powdered metal oxides such as well known compositions of yttria stabilized zirconia (YSZ) These TBCs are formed by heating a gas-propelled spray of the powdered oxide material using a plasma-spray torch, such as a DC plasma-spray torch, to a temperature at which the oxide powder particles become momentarily molten. The spray of the molten oxide particles is then directed onto a receiving metal surface or substrate, such as the surface of an article formed from a high temperature Ti-based, Ni-based, or Co-based superalloy, thereby forming a single layer of the TBC. In order to make TBCs having the necessary thicknesses, the process is repeated so as to deposit a plurality of individual layers on the surface of interest. Typical overall thicknesses of finished TBCs are in the range of approximately 0.010-0.055 inches.
The microstructure of a typical prior art TBC formed by APS deposition is described now by reference to FIGS. 1a and 1b. FIGS. 1a and 1b are scanning electron microscope (SEM) photomicrographs of fracture surfaces through the thickness of a prior art TBC taken at magnifications of 50.times. and 3000.times., respectively. The TBC has been removed by acid dissolution of the metal article on which it was deposited, and fractured to expose the characteristics of the resulting microstructure.
In order to make the TBC of FIGS. 1a and 1b, the TBC was deposited using an apparatus comprising an air plasma spray torch positioned adjacent to a rotatable cylindrical metal drum for holding the articles to be coated. The plasma spray torch was positioned at a distance from the drum and perpendicular to its axis such that it could be moved along a line parallel to the axis. A TBC was deposited by rotating the drum containing a metal article, comprising an approximately 0.125 inch thick coupon of a Ni-based alloy, while the plasma spray torch was moved in a path parallel to the drum axis, so as to make one pass across the exposed top surface of the metal coupon. Each rotation of the drum carried the plasma-spray torch onto, across and off the top surface of the coupon and resulted in the deposition of what is termed herein as a "single sub-layer" or simply a "sub-layer" of the TBC. The "spray pattern" or "footprint" of the torch deposit as termed herein, is a cross-section of the spray pattern of molten particles having a finite size, e.g. one-half inch in diameter. The footprint may be circular or other shapes depending on the shape of the plasma-spray stream, the angle of the surface of the article being deposited to the stream, and other factors. The size of the footprint is largely a function of the distance of the article from the plasma-spray gun and the shape of the plasma-spray stream. Depending on the combination of drum rotation rate and torch traverse rate, multiple sub-layers may be deposited at a given spot as the torch footprint passes over in a single pass. Therefore, a "primary layer", as termed herein, comprises the thickness of TBC of coating deposited in a single pass of the torch and may, and most often does, consist of a plurality of sub-layers. A "torch holiday", as termed herein, occurs when the plasma-spray torch from which a TBC is being deposited moves away from the area on the article on which the TBC is being deposited so that cooling of the surface occurs, or when the article is moved out from under the plasma-spray torch, or when the motion of both the article and the torch causes the area being deposited to be moved away from the stream of plasma-sprayed particles.
Referring to FIGS. 1a and 1b, the TBC was deposited in multiple passes, wherein the plasma spray torch was translated back and forth across the top surface of the coupon. During the passes, the drum upon which the coupon was secured was also rotated at a speed such that each area of the coupon being deposited with the TBC passed under the plasma-spray torch footprint a plurality of times during each pass, for example 4 to 5 times. This method of deposition produced layers in two respects, a primary layer resulted from each repeated translation of the torch across the surface of the substrate, secondary or sub-layers resulted from multiple rotations of the drum. In FIGS. 1a and 1b, the TBC includes about 150 primary layers resulting from the combination of the rotation of the drum and the translation of the torch.
The TBC shown in FIGS. 1a and 1b was made from -120 mesh YSZ powder having a composition of 8% yttria by weight with a balance of zirconia, and was deposited using a perimeter feed DC plasma spray torch, Model 7MB made by Metco Inc. The torch current was approximately 500 A, and the distance of the plasma spray flame to the surface of the article was approximately 3-5 inches. The deposition temperature measured at the back surface of the coupon was less than 260.degree. C. The resulting TBC was approximately 0.050 in. thick. Applicants believe that the TBC shown in FIGS. 1a and 1b is representative of prior art TBC generally.
FIG. 1a reveals a rough and irregular fracture surface, the reasons for which are more readily apparent from examination of FIG. 1b. The fracture surface of FIG. 1b is made up of what appears to be a stack of many discrete particles which do not share a common fracture plane, but which are rather fractured jaggedly along a path of what appears to have been weaker points within and between the individual particles. This jagged fracture path explains the rough appearance at the lower magnification of FIG. 1a. The explanation for the appearance of this fracture surface is given below.
As noted above, the TBC comprises a plurality of layers as a result of the combination of rotation of the drum and translation of the torch and area of the torch footprint. These layers are formed from the stream of individual molten particles of YSZ, which impact either the surface of the coupon, or particles from a previously deposited TBC layer. Upon impact, molten particles are joined to the metal article in part by a physical mechanical interlocking of the molten particles within the features provided by the surface roughness of the article, or to previously deposited particles by a process known as micro-welding, which is described further below. Applicants have observed in FIG. 1b, and in the examination of similar prior art TBCs, that the majority of these particles appear to be weakly bonded to particles in prior and subsequent sub-layers, and that micro-welding between sub-layers appears to be very limited; as evidenced by the distinct surfaces which still appear as demarcations between these sub-layers, such as are shown in FIG. 1b.
Referring to FIG. 1b, the particles appear as irregularly shaped platelets, and exhibit internally a fine-grained, columnar structure which is formed in a direction generally perpendicular to the contact surface of the underlying platelet or platelets (arrow 10 points in the direction of the outer surface of the TBC). Limited micro-welding between particles is indicated by the lack of a continuous, columnar grain structure between adjacent sub-layers. The lack of micro-welding results in an irregular, randomly oriented microstructure within the YSZ having the general appearance of compressed popcorn or polystyrene beads. Applicants believe that such a microstructure results because the combination of the heat contained within the molten powder particles and the heat contained on the deposition surface during the deposition is not sufficient to cause localized re-melting under the area where one particle impacts a previously deposited particle, resulting in limited or non-existent micro-welding between the deposited particles, and hence between sub-layers.
Limited micro-welding, as seen in FIGS. 1a and 1b, also results in a microstructure that exhibits a significant amount of both horizontal and vertical cracks, i.e. cracks oriented parallel to and normal to the substrate interface, respectively, surrounding such particles. For example, referring again to FIG. 1b, it will be further observed that some of the impacted particles have what appear to be gaps or separations between them.
Applicants have observed that even when the micro-welding between individual particles has been improved such that columnar grain growth occurs continuously between individual particles, such continuous columnar growth does not extend coherently (as described further below) across the boundaries between the layers that comprise prior art TBCs. Thus, while some columnar ordering of adjacent particle sub-layers comprising the microstructure of prior art TBCs may occur, this ordering is limited, and the lack of coherency between layers often results in horizontal cracking in the regions between layers for the same reasons as discussed above. In fact, a low deposition surface temperature (due to the torch holiday which defines a layer) during the deposition of either sub-layers or layers decreases the likelihood that micro-welding will occur and increases the potential for creation of both horizontal and vertical cracks during the deposition. Therefore, cracking which occurs between layers may be even more severe, and result in horizontal macrocracks (cracks which extend over distances that are substantially larger than the diameter of an individual particle).
One well recognized problem in the use of prior art TBC coatings, particularly on articles routinely cycled from ambient conditions up to extremely high temperatures such as those used in gas turbines, is that the exposure of TBCs to the very intense heat and rapid temperature changes associated with high velocity combustion gases can cause their failure by spallation, or spalling of the TBC from the surfaces of the metal articles which they are designed to protect, possibly due to thermal fatigue. Susceptibility to spallation in cyclic thermal environments is primarily due to the existence of horizontal cracking or in-plane (of the TBC) cracking. Horizontal cracks are known particularly to increase the susceptibility of a TBC to spallation because in-plane stresses, such as in-plane stresses created during the TBC deposition process or in service, can cause such horizontal cracks to propagate and grow.
It is known that the spallation resistance of TBCs in such environments can be improved by modifying certain characteristics of the coatings. For example, in the article entitled: "Experimental and Theoretical Aspects of Thick Thermal Barrier Coatings for Turbine Applications"; V. Wilms, G. Johner, K. K. Schweitzer and P. Adams; THERMAL SPRAY: Advances in Coatings Technology; Proceedings of the National Thermal Spray Conference; Orlando, Fla.; September 1987; pp. 155-166 it is disclosed that the performance of yttria stabilized zirconia (YSZ) TBCs is enhanced in cyclic thermal environments by developing a predominance of cracks normal to the TBC/metal article interface (i.e. vertical cracks) and a minimum of cracks parallel to such interface (i.e. horizontal cracks). Also, U.S. Pat. No. 5,073,433 issued to Taylor teaches that the existence of homogeneously dispersed vertical macrocracking with a controlled amount of horizontal cracking within a TBC reduces the tendency for spalling within the coating, and thus increases the thermal fatigue resistance. However, this patent does not teach any associated microstructural improvements in such TBCs, such as improved micro-welding of adjacent particle sub-layers as described hereinbelow. In fact, U.S. Pat. No. 5,073,433 teaches the necessity of controlling such horizontal cracking.
Applicants have observed that it is possible to develop a preferred vertical macrocrack pattern, as described in U.S. Pat. No. 5,073,433, without otherwise substantially altering the prior art microstructure as described above. A TBC containing vertical macrocracks, horizontal cracks and horizontal microcracks is shown in FIGS. 2a and 2b. FIG. 2a is an optical photomicrograph at 50.times. magnification of a polished cross-section of a prior art TBC (arrow 20 points in the direction of the outer surface of the TBC) which reveals the presence of preferred vertical macrocracks as described in U.S. Pat. No. 5,073,433. However, FIG. 2b which is an electron photomicrograph of a fracture surface of the same coating taken at 2000.times., reveals a prior art microstructure similar to that described for FIGS. 1a and 1b, although the individual particles are not as evident in FIG. 2b. However, no long range ordering of the columnar grains is apparent, particularly ordering that would extend beyond the thickness of a single layer which is about 0.0004-0.0005 inches. The approximate thickness of a single deposition layer for this TBC is illustrated by vertical bar 30 for comparison. FIGS. 2a and 2b also reveal the presence of a substantial amount of horizontal macrocracks and microcracks. The TBC shown in FIGS. 2a and 2b was also deposited using the apparatus and method described above for the TBC shown in FIGS. 1a and 1b, under similar conditions. Therefore, it may be seen that it is possible to develop preferred vertical or segmentation cracking in a TBC having substantial undesirable horizontal cracking, due to the existence of a prior art microstructure which does not exhibit sufficient micro-welding, either within or between layers and/or sub-layers, to establish a coherent, continuous columnar grain structure.
Therefore, Applicants have observed that the tendency for spallation in cyclic, high temperature environments which is known to exist in prior art TBCs is related directly to weak or non-existent micro-welding between adjacent particle sub-layers due to a lack of continuous columnar grain growth, particularly between TBC layers, as explained above. Therefore, it is desirable to improve the microstructure of TBCs by improving micro-welding and reducing the amount of horizontal cracking. Applicants herein identify such improved TBCs and their microstructural characteristics.